Biochemical process for optical resolution of cyclopentenolone derivatives

ABSTRACT

A biochemical process for optical resolution of 4-hydroxy-3-methyl-2-2&#39;-propynyl-2-cyclopentenone, which comprises asymmetrically hydrolyzing organic carboxylic acid (saturated or unsaturated carboxylic acids having 1 to 18 carbon atoms) esters of (±)-4-hydroxy-3-methyl-2-2&#39;-propynyl-2-cyclopentenone by the action of an esterase originated from a microorganism or an animal pancreas to give optically active 4-hydroxy-3-methyl-2-2&#39;-propynyl-2-cyclopentenone and esters of its antipode with high optical purity is disclosed. This resolution process is simple in steps as compared with the conventionally known organic chemical methods of optical resolution, and is quite advantageous industrially because it does not require expensive optically active reagents.

THE TECHNICAL FIELD

This invention relates to a biochemical process for optical resolutionof (±)-4-hydroxy-3-methyl-2-2'-propynyl-2-cyclopentenone represented bythe following formula (I): ##STR1## More particularly, the inventionrelates to an industrially advantageous biochemical process for opticalresolution of 4-hydroxy-3-methyl-2-2'-propynyl-2-cyclopentenone, whichcomprises asymmetrically hydrolyzing organic carboxylic acid (saturatedor unsaturated carboxylic acids having 1 to 18 carbon atoms) esters of(±)-4-hydroxy-3-methyl-2-2'-propynyl-2-cyclopentenone by the action ofan esterase originated from a microorganism or an animal pancreas togive optically active 4-hydroxy-3-methyl-2-2'-propynyl-2-cyclopentenoneand esters of its antipode with high optical purity.

BACKGROUND TECHNOLOGY

4-Hydroxy-3-methyl-2-2'-propynyl-2-cyclopentenone (hereinafter referredto as propynylcyclopentenolone) is known as one of the important alcoholcomponents in a group of ester compounds called "synthetic pyrethroids"which have excellent insecticidal activities.

For example, a compound of the formula (II) as described below which isan ester of the propynylcyclopentenolone and2,2,3,3-tetramethylcyclopropanecarboxylic acid is an excellentinsecticide having quite strong knockdown and lethal effects (JapanesePublished Examined Patent Application No. 50-15843). ##STR2##

There are two kinds of optical isomers in the propynylcyclopentenolonebecause it has an asymmetric carbon atom in the 4-position thereof, andusually, their activities as esters are greatly different. For example,in the ester represented by the foregoing formula (II), an ester of the(+)-propynylcyclopentenolone has an insecticidal activity several timeshigher than that of a corresponding ester of the(-)-propynylcyclopentenolone. Thus, there are demanded industriallyadvantageous technologies of optical resolution of the(±)-propynylcyclopentenolone obtained by the conventional productionprocesses.

As methods for producing optically active propynylcyclopentenolone, afollowing method has been known; which method comprises converting(±)-propynylcyclopentenolone into a half ester of phthalic acid,reacting this ester with an optically active amine to form diastereomersalts of the optically active propynylcyclopentenolone, isolating andrecovering a salt as the amine used and half ester, and then,hydrolyzing the resulting half ester (Japanese Published UnexaminedPatent Application No. 56-2929). It is hard to say that this process issatisfactory, however, because it is low in overall yield and requirescomplicated procedures and expensive optically active reagents.

DISCLOSURE OF THE INVENTION

The present inventors have conducted extensive studies in order toestablish a process for optical resolution of(±)-propynylcyclopentenolone, which may overcome the foregoing problemsand is more advantageous industrially. As a result, the presentinventors have found that optically active propynylcyclopentenolone andesters of its antipode having high optical purity can be obtained byreacting the organic carboxylic acid (saturated or unsaturated organiccarboxylic acids having 1 to 18 carbon atoms) esters of(±)-propynylcyclopentenolone with an esterase originated from amicroorganism or an animal pancreas. The present invention has beenachieved by giving various considerations to such finding.

Hereunder, the present invention will be explained in detail. Organiccarboxylic acid (saturated carboxylic acids having 1 to 18 carbon atoms)esters of (±)-propynylcyclopentenolone used as the starting material inthe process of the present invention can be easily prepared byconventional methods for the production of esters, e.g., a method forreacting (±)-propynylcyclopentenolone with anhydrides of organiccarboxylic acids, a method for reacting it with organic carboxylic acidchlorides in the presence of organic bases, or the like.

The microorganisms, which produce employable esterase in the invention,are those capable of asymmetrically hydrolyzing the organic carboxylicacid esters of (±)-propynylcyclopentenolone and are not particularlyrestricted in the origin. (In this specification, the term "esterase"means an esterase in a broad sense, including lipase.) As the examplesof such microorganisms, the following genuses may be illustrated:Enterobacter, Arthrobacter, Brevibacterium, Pseudomonas, Alcaligenes,Flavobacterium, Micrococcus, Chromobacterium, Mycobacterium,Corynebacterium, Bacillus, Lactobacillus, Trichoderma, Candida,Saccharomyces, Rhodotorula, Cryptococcus, Torulopsis, Pihia,Penicillium, Aspergillus, Rhizopus, Mucor, Aureobasidium, Actinomucor,Nocardia, Streptomyces.

Names of typical strains belonging to the respective genera areillustrated below, but the microorganisms of the present invention arenot limited to these illustrative examples.

    ______________________________________                                        (1)    Enterobacter cloacae                                                                              IFO     3320                                       (2)    Arthrobacter simplex                                                                              IFO     3530                                       (3)    Brevibacterium ammoniagenes                                                                       IFO     12072                                      (4)    Pseudomonas fluorescens                                                                           IFO     3081                                       (5)    Alcaligenes faecalis                                                                              IFO     12669                                      (6)    Flavobacterium arborescens                                                                        IFO     3750                                       (7)    Micrococcus luteus  OUT     8276                                       (8)    Chromobacterium viscosum                                                                          ATCC    6918                                       (9)    Mycobacterium phlei IFO     3158                                       (10)   Corynebacterium equi                                                                              ATCC    7699                                       (11)   Bacillus subtilis   IFO     3026                                       (12)   Lactobacillus casei IFO     3322                                       (13)   Trichoderma viride  IFO     4847                                       (14)   Saccharomyces rouxii                                                                              IFO     0505                                       (15)   Candida utilis      IFO     0396                                       (16)   Rhodotorula minuta  IFO     0879                                       (17)   Cryptococcus albidus                                                                              IFO     0378                                       (18)   Torulopsis candida  IFO     0768                                       (19)   Pihia polimorpha    IFO     1166                                       (20)   Penicillium frequentans                                                                           IFO     5692                                       (21)   Aspergillus var asper                                                                             IFO     5324                                       (22)   Rhizopus chinensis  IFO     4737                                       (23)   Aureobasidium pullulans                                                                           IFO     4464                                       (24)   Actinomucor elegans IFO     4022                                       (25)   Nocardia asteroides IFO     3424                                       (26)   Streptomyces griseus                                                                              IFO     3356                                       (27)   Mucor javanicus     IFO     4572                                       ______________________________________                                    

All of these strains are deposited in American Type Culture Collection(ATCC), Osaka University, Faculty of Engineering, Department ofFermentation Technology (OUT), or Institute of Fermentation, Osaka (IFO)and are available from these depositories.

Such microorganisms may be cultured in a liquid medium to obtain acultured medium according to the conventional procedures. For example, amicroorganism may be inoculated to a sterilized liquid medium [for fungiand yeasts, a malt extract-yeast extract medium (5.0 g of peptone, 10.0g of glucose, 3.0 g of malt extract and 3.0 g of yeast extract dissolvedin 1 l of water and adjusted to pH 6.5), and for bacteria, a sugar-addedbouillon medium (10.0 g of glucose, 5.0 g of peptone, 5.0 g of meatextract and 3.0 g of NaCl dissolved in 1 l of water and adjusted to pH7.2)], and subjected to a shaken culture ordinarily at a temperature of20° to 40° C. for 2 to 3 days. In addition, if necessary, it may becultured on a solid medium.

In the present invention, among the above-described microorganisms,those belonging to the genera Enterobacter, Arthrobacter,Brevibacterium, Pseudomonas, Alcaligenes, Chromobacterium,Mycobacterium, Bacillus, Trichoderma, Candida, Rhodotorula, Torulopsis,Aspergillus, Rhizopus, Mucor, Nocardia and Streptomyces are particularlypreferable in view of the esterase activity and asymmetric yield.

Some of the esterases originated from these microorganisms arecommercially available. Specific examples of such esterases include alipase from the Pseudomonas sp. (made by Amano Seiyaku Co., Ltd.),lipase from the Aspergillus sp. (Lipase AP, made by Amano Seiyaku Co.,Ltd.), lipase from the Mucor sp. (Lipase M-AP, made by Amano SeiyakuCo., Ltd.), lipase from Candida cylindracea (Lipase MY, made by MeitoSangyo Co., Ltd.), lipase from the Alcaligenes sp. (Lipase PL, made byMeito Sangyo Co., Ltd.), lipase from the Achromobacter sp. (Lipase AL,made by Meito Sangyo Co., Ltd.), lipase from the Arthrobacter sp.(Lipase Godo BSL, made by Godo Shusei Co., Ltd.), lipase from theChromobacterium sp. (made by Toyo Jozo Co., Ltd.), lipase from Rhizopusdelemar (Talipase, made by Tanabe Seiyaku Co., Ltd.) and lipase from theRhizopus sp. (Lipase Saiken, made by Osaka Saikin Kenkyusho).

In addition, steapsin and pancreatin can be used as the animal-originpancreatic esterase.

THE BEST MODE FOR EMBODIMENT OF THE INVENTION

In practicing the process of the present invention, the asymmetrichydrolysis of organic carboxylic acid (saturated or unsaturatedcarboxylic acids having 1 to 18 carbon atoms) ester of(±)-propynylcyclopentenolone is carried out by stirring or shaking amixture of said ester and an esterase-containing liquor, such ascultured liquor of such microorganisms, microorganism cells separatedfrom the cultured liquor, esterase-containing culture filtrates, crudeor purified esterases or esterase-containing extracts or concentratesseparated from microorganism cells or culture filtrates by variousenzymatic separation procedures, or aqueous solution containing esteraseoriginated from animal pancrease. Alternatively, the microorganism cellor the esterase may be used in an immobilized state.

For the conditions of conducting the asymmetric hydrolysis of theorganic carboxylic acid esters of (±)-propynylcyclopentenolone, asuitable reaction temperature is 10° to 70° C., preferably 50° to 65° C.for cultured media of thermophilic bacteria or thermostable esterasesobtained by culture of thermophilic bacteria, and 20° to 50° C. forcultured media of mesophilic bacteria or esterases having nothermostability.

The reaction time is usually 3 to 48 hours, but it can be shortened byelevating the reaction temperature or by increasing the amount ofesterases.

It is preferred that the pH value during the reaction is 8 to 11 forcultured media of alkalophilic bacteria or alkaline esterases, and 5 to8 for cultured media of non-alkalophilic microorganisms or esteraseshaving no stability to alkali. In addition, use of buffer solutions ispreferred in order to neutralize the organic carboxylic acids formed bythe hydrolysis to keep the pH value during the reaction constant. Buffersolutions of inorganic acid salts such as sodium phosphate, potassiumphosphate, etc., and those of organic acid salts such as sodium acetate,sodium citrate, etc., can be used.

The concentration of the organic carboxylic acid esters of(±)-propynylcyclopentenolone used for the substrate is 1 to 50 wt%,preferably 5 to 25 wt%, to the reaction mixture.

After such asymmetric hydrolysis reaction, obtained optically activefree propynylcyclopentenolone and unreacted antipode ester are separatedfrom each other and recovered. In the separating and recovering, suchprocedures can be employed as steam distillation, solvent extraction,fractional distillation, column chromatography, and the like. Forexample, the reaction mixture is steam distilled, followed by extractionof the distillate with diethyl ether, or directly extracted with organicsolvents such as diethyl ether, ethyl acetate, benzene, etc. The extractis then subjected to fractional distillation to isolate optically activepropynylcyclopentenolone from an ester of its antipode, or to silica gelcolumn chromatography. The chromatography procedures are carried out by,for example, first isolating an organic carboxylic acid ester ofoptically active propynylcyclopentenolone by eluting with atoluene-ethyl acetate (5:1) solution, and then isolating the antipodefree propynylcyclopentenolone by eluting with a toluene-ethyl acetate(2:1) solution.

The ester of optically active propynylcyclopentenolone thus separatedcan be easily introduced into optically active propynylcyclopentenoloneby further deacylation. The reaction of deacylation is carried out by,for example, adding water and an equivalent amount of sodiumhydrogencarbonate to the organic carboxylic acid ester ofpropynylcyclopentenolone, further adding a 10% HCl aqueous solution toadjust to pH 5, and subsequently refluxing with stirring for 8 hours,whereby propynylcyclopentenolone can be readily obtained.

As described above in detail, the optical resolution of(±)-propynylcyclopentenolone according to the process of the presentinvention is simple in steps as compared with the conventional method oforganic chemical optical resolution, and is economically advantageousbecause of its no requirement for expensive optically active reagents.

Moreover, the process of the invention is quite advantageousindustrially because it gives optically active propynylcyclopentenolonewith high yield and optical purity.

The present invention will be described in further detail by referenceto the following examples, but it is not limited thereto.

EXAMPLES 1 TO 8

1.0 g of an acetic acid ester of (±)-propynylcyclopentenolone and 20 mgof each esterase as shown in Table 1 were added to 10 ml of a buffersolution (McIlvaine buffer solution of pH 7.0 or 0.2M Na₂ CO₃ --NaHCO₃buffer solution of pH 9.5) and allowed to react at 30° C. with vigorousstirring using an agitator. After 24 hours of the reaction, the reactionproduct was extracted with ethyl acetate. The extract was analyzed bygas chromatography (5% DEGS, 1.1 m, 180° C.), and a hydrolysis rate wascalculated from the peak area ratio of propynylcyclopentenolone to itsacetic acie ester. The results are shown below. The extract wasconcentrated, subjected to silica gel column chromatography and elutedwith a toluene-ethyl acetate (5:1) solution to isolate an unreactedacetic acid ester of propynylcyclopentenolone, which was then furthereluted with a toluene-ethyl acetate (2:1) solution to obtain freepropynylcyclopentenolone.

10 mg of the free propynylcyclopentenolone thus obtained was dissolvedin 1 ml of toluene, and 0.2 ml of pyridine and 45 mg of(-)-α-methoxy-α-trifluoromethylphenylacetic chloride ((-)-MTPA chloride)were added thereto, followed by refluxing under heating for 1 hour toobtain a (-)-MTPA-diastereomer of propynylcyclopentenolone. This wasthen subjected to optical isomeric analysis by gas chromatography(Silicon DCQF-1, 30 m capillary columns, 180° C.), and the opticalisomer form and optical purity of the free propynylcyclopentenolone weredetermined from the peak area ratio of the diastereomer of(+)-propynylcyclopentenolone to that of (-)-propynylcyclopentenolone.

Meanwhile, to the unreacted ester obtained by the column chromatographyprocedure were added 5 ml of water and an equivalent amount, to theester, of sodium hydrogencarbonate, and further added a 10% HCl aqueoussolution to adjust to pH 5, followed by deacylating by refluxing withstirring for 8 hours. After cooling, ethyl acetate, water and NaCl wereadded to the reaction mixture to thereby effect extraction. Afterdistilling away the solvent, a thus-obtained oily material was subjectedto silica gel column chromatography and eluted with a toluene-ethylacetate (2:1) solution to obtain optically activepropynylcyclopentenolone.

The propynylcyclopentenolone was analyzed for the optical isomer ratioby gas chromatography as described above to determine the optical isomerform and optical purity of the unreacted ester. The results are shown inTable 1.

                                      TABLE 1                                     __________________________________________________________________________                         Free                                                                          Propynylcyclo-                                                                pentenolone                                                                           Unreacted Ester                                                 Hydrolysis                                                                          Optical                                                                           Optical                                                                           Optical                                                                           Optical                                      Example                                                                            Origin of Rate  Isomer                                                                            Purity                                                                            Isomer                                                                            Purity                                       No.  Esterase  (%)   Form                                                                              (%) Form                                                                              (%)                                          __________________________________________________________________________    1    Arthrobacter sp.                                                                        50.0  (-) 100 (+) 100                                               (Lipase Godo BSL)                                                        2    Pseudomonas sp.                                                                         50.0  (-) 100 (+) 100                                          3    Achromobacter sp.                                                                       50.9  (-) 96.5                                                                              (+) 100                                               (Lipase AL)                                                              4    Alcaligenes sp.                                                                         54.1  (-) 84.8                                                                              (+) 100                                               (Lipase PL)                                                              5    Aspergillus sp.                                                                         47.6  (-) 87.6                                                                              (+) 78.6                                              (Lipase AP)                                                              6    Mucor sp. 43.5  (-) 95.5                                                                              (+) 70.0                                              (Lipase M-AP)                                                            7    Rhizopus sp.                                                                            40.5  (-) 85.0                                                                              (+) 57.5                                              (Lipase Saiken)                                                          8    Porcine Pancreas                                                                        49.6  (-) 100 (+) 99.0                                              (steapsin)                                                               __________________________________________________________________________

EXAMPLES 9 TO 15

0.65 g of a capric ester of (±)-propynylcyclopentenolone and 10 mg ofeach esterase as shown in Table 2 were added to 5 ml of a buffersolution and allowed to react at 30° C. for 24 hours with vigorousstirring using an agitator. Subsequent procedures were similar to thosein Examples 1 to 8. The results are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                           Free                                                                          Propynylcyclo-                                                                pentenolone                                                                           Unreacted Ester                                                 Hydrolysis                                                                          Optical                                                                           Optical                                                                           Optical                                                                           Optical                                    Example          Rate  Isomer                                                                            Purity                                                                            Isomer                                                                            Purity                                     No.  Origin of Esterase                                                                        (%)   Form                                                                              (%) Form                                                                              (%)                                        __________________________________________________________________________     9   The Arthrobacter sp.                                                                      50.0  (-) 100 (+) 100                                             (Lipase Godo BSL)                                                        10   The Alcaligenes sp.                                                                       50.0  (-) 100 (+) 100                                             (Lipase PL)                                                              11   The Achromobacter sp.                                                                     50.0  (-) 100 (+) 100                                             (Lipase AL)                                                              12   The Pseudomonas sp.                                                                       47.1  (-) 100 (+) 88.4                                       13   The Aspergillus sp.                                                                       38.8  (-) 99.5                                                                              (+) 60.2                                            (Lipase AP)                                                              14   Rhizopus delemar                                                                          35.6  (-) 83.0                                                                              (+) 50.0                                            (Talipase)                                                               15   Porcine Pancreas                                                                          51.1  (-) 98.7                                                                              (+) 100                                             (Steapsin)                                                               __________________________________________________________________________

EXAMPLES 16 TO 21

In a 500 ml flask with shoulders was placed 100 ml of a liquid medium(100 ml) [for fungi and yeasts (Examples 20 and 21), a maltextract-yeast extract medium (5.0 g of peptone, 10.0 g of glucose, 3.0 gof malt extract and 3.0 g of yeast extract dissolved in 1 l of water andadjusted to pH 6.5), and for bacteria (Examples 16 to 19), a sugar-addedbouillon medium (10.0 g of glucose, 5.0 g of peptone, 5.0 g of meatextract and 3.0 g of NaCl dissolved in 1 l of water and adjusted to pH7.2)]. After sterilization, the resulting medium was inoculated with twoplatinum loop-fuls of each slant cultured microorganism as shown inTable 3, and cultured on a reciprocating shaker at 30° C. for 48 hours.Then, to this cultured medium was added 7 g of an acetic acid ester of(± )-propynylcyclopentenolone, followed by reciprocal shaking at 30° C.for 24 hours. Subsequently, the reaction mixture was steam distilled,and the distillate was extracted with diethyl ether. The extract wasconcentrated, and free propynylcyclopentenolone was separated by thesame column chromatography procedure as in Examples 1 to 8. The freepropynylcyclopentenolone was subjected to optical isomeric analysis bythe same procedure as in Examples 1 to 8. The results are shown in Table3.

                  TABLE 3                                                         ______________________________________                                                                 Free Propynyl-                                                                cyclopenten-                                                           Hydrol-                                                                              olone                                                Exam-                   ysis     Optical                                                                             Optical                                ple                     Rate     Isomer                                                                              Purity                                 No.   Microorganisms Used                                                                             (%)      Form  (%)                                    ______________________________________                                        16    Arthrobacter simplex                                                                            49.0     (-)   97.0                                         IFO 3530                                                                17    Pseudomonas fluorescens                                                                         49.0     (-)   100                                          IFO 3081                                                                18    Nocardia asteroides                                                                             53.9     (-)   86.5                                         IFO 3424                                                                19    Brevibacterium ammoniagenes                                                                     30.9     (+)   89.1                                         IFO 12072                                                               20    Rhodotorula minuta                                                                              45.3     (-)   90.2                                         IFO 0879                                                                21    Trichoderma viride                                                                              50.2     (-)   100                                          IFO 4847                                                                ______________________________________                                    

EXAMPLE 22

One liter (1 l) of a cultured medium of Trichoderma viride IFO-4847 asprepared by the same procedure as in Example 21 was filtered to obtain aculture filtrate. 30 ml of the culture filtrate was concentrated toone-third, and 1.5 g of a formic acid ester of (±)-propynylpentenolonewas added thereto, followed by stirring vigorously at 30° C. for 30hours using an agitator. Subsequent procedures were similar to those inExamples 1 to 8. The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                 Free                                                                          Propynylcyclo-                                                                pentenolone  Unreacted Ester                                         Hydrolysis Optical Optical    Optical                                                                             Optical                                   Rate       Isomer  Purity     Isomer                                                                              Purity                                    (%)        Form    (%)        Form  (%)                                       ______________________________________                                        50.0       (-)     100        (+)   100                                       ______________________________________                                    

EXAMPLE 23

Microorganism cells were collected by centrifugal separation from 1 l ofa cultured medium of Rhodotorula minuta IFO 0879 as prepared by the sameprocedure as in Example 20, washed twice with distilled water, and thenlyophilized. 500 mg of the freeze-dried microorganism cells and 1 g of aformic acid ester of (±)-propynylcyclopentenolone were added to 10 ml ofa McIlvaine buffer solution of pH 7.0, followed by stirring vigorouslyat 30° C. for 30 hours using an agitator. Subsequent procedures weresimilar to those in Examples 1 to 8. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                 Free                                                                          Propynylcyclo-                                                                pentenolone  Unreacted Ester                                         Hydrolysis Optical Optical    Optical                                                                             Optical                                   Rate       Isomer  Purity     Isomer                                                                              Purity                                    (%)        Form    (%)        Form  (%)                                       ______________________________________                                        47.3       (-)     100        (+)   89.0                                      ______________________________________                                    

What is claimed is:
 1. A biochemical process for optical resolution of(±)-4-hydroxy-3-methyl-2-2'-propynyl-2-cyclopentenone, characterized inthat an organic carboxylic acid (saturated or unsaturated carboxylicacids having 1 to 18 carbon atoms) ester of(±)-4-hydroxy-3-methyl-2-2'-propynyl-2-cyclopentenone is reacted with anesterase derived from a microorganism or an animal pancreas, so as toasymmetrically hydrolyze said ester into optically active4-hydroxy-3-methyl-2-2'-propynyl-2-cyclopentenone and esters of itsantipode.
 2. A process according to claim 1, wherein the esterase isderived from a microorganism belonging to the genus Enterobacter,Arthrobacter, Brevibacterium, Pseudomonas, Alcaligenes, Flavobacterium,Micrococcus, Chromobacterium, Mycobacterium, Corynebacterium, Bacillus,Lactobacillus, Trichoderma, Candida, Saccharomyces, Rhodotorula,Cryptococcus, Torulopsis, Pihia, Penicillium, Aspergillus, Rhizopus,Mucor, Aureobasidium, Actinomucor, Nocardia, Streptomyces, orAchromobacter.
 3. A process according to claim 1, wherein the esteraseis derived from a microorganism belonging to the genus Enterobacter,Arthrobacter, Brevibacterium, Pseudomonas, Alcaligenes, Chromobacterium,Mycobacterium, Bacillus, Trichoderma, Candida, Rhodotorula, Torulopsis,Aspergillus, Rhizopus, Mucor, Nocardia, Streptomyces, or Achromobacter.4. A process according to claim 1, wherein the organic carboxylic acidester is an ester of acetic acid, capric acid, or formic acid.
 5. Aprocess according to claim 1, wherein the organic carboxylic acid esteris an ester of acetic acid.